1. Field of the Invention
The invention concerns detectors used to detect radiation with different energies and, notably, detectors of X-rays with at least two different energy levels.
2. Description of the Prior Art
X-rays are commonly used in radiology to examine materials because their attenuation depends on their energy and on a coefficient which is characteristic of the material examined for this energy The X-rays that are given by the emission tubes have a fairly extensive energy spectrum so that the radiation transmitted by the materials examined contains a wide variety of information on said materials. A part of this diversity is lost when the radiation transmitted is applied to a single detector which has a defined function of sensitivity depending on the energy. Thus, with a single detector, the fact that high energy levels are more representative of absorption by heavy materials cannot be used to reveal, in the transmitted radiation, the nature of the materials transmitted. On the contrary, if two detectors are placed in the path of the transmitted rays, said two detectors having maximum sensitivity levels for different energy values, one of the energy values being more representative of one of the constituents of the material examined, it then becomes possible, after weighting the detected signal by suitable coefficients and subtracting said weighted signals, to remove the effect of this material. Thus, in a medical picture of the thorax, it is possible to make the skeleton disappear and highlight other organs.
The simultaneous use of two detectors with different sensitivities is known, and detectors of this type are called "double energy" detectors, and are made, for example, by superimposing two detectors. A first detector is arranged to receive all the radiation transmitted so as to detect the low energy rays and transmit, to a second underlying detector, only those rays that have greater energy. To this effect, the first detector is planned for energy of 20 to 30 kev and uses, for example, an yttrium oxysulphide or an oxide, sometimes a lanthanum oxysulphide, while the latter is designed for energy values of 40 to 100 kev and uses, for example, gadolinium oxysuphide, calcium tungstate or cadmium tungstate.
To enable a radiological system having a double energy detector to work properly, it is necessary to obtain and maintain, for all the angles of incidence, a perfect alignment of the focus of the X-ray tube, the element observed, and the two detectors with equal precision to within the dimension of the smallest element that can be observed. This geometrical imperative is difficult to achieve and maintain over a period of time, and is expensive. Furthermore, in the case of X-ray tubes with rotating anodes, it is also necessary to take into account a certain degree of fluctuation in the original point of the X-rays.
In another embodiment of the prior art, the two detectors are placed side by side. In this case, in the processing of the signals received from each detector, it is necessary to take into account the divergence between the two detectors which do not perceive the same thing at the same instant. Furthermore, even by performing this divergence correction, there is nonetheless a source of error which remains, because the parts of the human body which are examined do not stay completely still during the period of movement corresponding to the divergence of the detectors.